A door locking system that unlocks or locks doors of a vehicle if a user who carries a communication device (vehicle key) associated with the vehicle operates an operation unit provided in an outer surface of a door of the vehicle, also known as a so-called smart entry system is installed in vehicles. Furthermore, a door locking system that unlocks or locks doors of a vehicle if a user operates an operation unit (an unlocking button or a locking button) provided in a communication device, also known as a so-called keyless entry system is installed in vehicles.
In general, these door locking systems are configured to unlock all doors if the doors are in a locked state when the operation unit of the vehicle or the operation unit of the communication device is operated, and lock all doors if the doors are in an unlocked state.
Door locking systems have also been proposed that determine, when unlocking doors of a vehicle, the position where a communication device is present relative to the vehicle, and unlocks only the door that corresponds to the position where the communication device is present (e.g., see JP2010-133099A (Patent Document 1)). In the system disclosed in Patent Document 1, if, for example, a user who carries the communication device operates an operation unit provided in a door on the driver seat side, only the door on the driver seat side is unlocked. Thus, since only the door near the user (communication device) is unlocked, a suspicious person who is hiding on the opposite side of the vehicle can be prevented from entering the vehicle from other doors.